Error Message: This is not a Genuine Copy of Windows

I have started getting the above error message after 2 years of using this computer. It is genuine, I have the original Windows operating disc and Activation code. How do I manage to reactivate windows?

Thanks - I did contact Microsoft and after being on the chat line with an agent in tech for 40 minutes, they told me I would have to pay $149 and buy their service package to fit it.
I think I will just format and reload, though I hate to think of all the wasted time.

The Following User Says Thank You to Dick-Y For This Useful Post:

Thank you - I got part way thru this but it was not downloading anything when I got to the download screen but will try it again later when I have more time. Sur hope it will work then as it would be much easier than a reinstall.
Charlene

The Following User Says Thank You to BruceR For This Useful Post:

I actually ran into this just yesterday. Randomly told me that a machine that was activated and working fine, suddenly was not "Genuine" Windows. I went and got the OD and re-entered the code into the activation box, and it worked again. Have you tried just re-entering your existing code?